In order to convert AC current to DC current of arbitrary voltage, it is necessary to perform rectification or step-up and step-down using certain methods. Since a pulsating current is obtained in the conversion described above, it is also necessary to smooth the pulsating current in order to have a smoother waveform in a bad case.
A simple smoothing method that has been used for a long time is a method of using an electrolytic capacitor having a large capacity after rectification. By causing the capacitor to be charged and discharged, a portion of the valley of the pulsating current is covered by the discharging from the capacitor.
The recent years, a power factor correction circuit using a switching technique has been used. This mainly controls a current flowing through the coil by switching. In a so-called capacitor input, type or condenser input type smoothing circuit that uses an electrolytic capacitor having a large capacity, charging until the voltage of the pulsating current reaches the apex and discharging when the pulsating voltage is reduced thereafter are repeated.
However, since discharging occurs immediately after charging, electric power of the capacitor is applied in a time zone in which the voltage drops from the apex of the voltage of the pulsating current. This is not the electric power from the AC power supply but is reactive power. A reduction in the power factor causes many problems, such as the generation of harmonics. This becomes a burden on the power transmission and distribution side. When harmonic current flows excessively through the power line, a problem, such as burnout of a condenser inserted in the power line, occurs in the worst case.
As a power factor correction circuit, there is a circuit abbreviated as PFC. However, this circuit is complicated, and power loss due to switching also occurs since the switching is accompanied. In addition, there is also a disadvantage in that switching noise is easily transmitted through the power supply line.
Thus, since a capacitor input type is simple, the number of components is small and the cost is low. On the other hand, there is a power factor problem. The power factor can be improved by the PFC circuit, but a complicated circuit configuration is required and accordingly the cost is high. In addition, a circuit for suppressing noise is also required.
In addition, related technologies were investigated, but any related technology corresponding to the so-called prior art was not found. For example, there is an invention disclosed in JP-A-2005-19266 that uses a PFC circuit. The invention includes a transformer having a power transformation function and a PFC control unit that causes the transformer to perform the transformation of DC power by the on/off control of a switching element in order to suppress a harmonic current. However, there is almost no relationship with the present invention.
[Patent Citation] JP-A-2005-19266